1. Field of the Invention
The present invention relates to a system for the remote testing of the transmission quality of a two-lead line which, under testing, links a measuring and testing apparatus at one end of the line to a load circuit at the other end of the line. More particularly, the system detects and locates failures in a telephone subscriber two-lead line or loop including a test device or quadrupole and which serves a subscriber installation. The measuring and testing apparatus is selectively connected to the subscriber loop through the respective switching facility or a local telephone exchange.
The line faults localized by the system fall into two types. The first type relates to breakdowns, also referred to as open circuits or cuts-off in one of the line leads. The second type covers short circuits also referred to as loops or shunts having a low resistance between the two-line leads or one of the line leads and ground.
2. Description of the Prior Art
In known test systems a test apparatus for locating line failures includes supply means. After the failures have been detected by a test desk or test rack that any switching facility has, they are located with respect to a test quadrupole. Depending on a response from the quadrupole subjected to the voltage or the current derived by the supply means, the test apparatus enables a fault to be located in the first line section between the switching facility and the test quadrupole or in the second line section between the test quadrupole and the subscriber installation. Generally, the second section is short and the test quadrupole is in the vicinity of the subscriber telephone installation. The location of a fault gives a guide as to whether the maintenance team that will be called upon to repair the line is ascribed to the first telephone line section at the switching facility end or to the second line section at the subscriber installation end. Indeed, the know-how and tooling required for troubleshooting the line or first section and the subscriber telephone installation or second section are most different. The location of a fault, rather than its precise nature, can consequently constitute a decision criterion when it comes to designating the maintenance team to be dispatched.
If an in-line fault is involved, the maintenance team is equipped with ladders and the gear needed to climb down into a chamber or the sewers. Were the fault is located at the subscriber end, then the maintenance team needs an appointment with the subscriber.
U.S. Pat. No. 4,054,759 describes two types of test quadrupoles or test circuits, each of which is interconnected in a two-lead telephone line or subscriber loop.
The first type of quadrupole comprises a relaxation oscillator and a relay first contact, the oscillator and contact are connected in series between the two-line leads. A connection means for connecting the oscillator to the subscriber loop in response to a DC predetermined signal includes the relay and a second relay contact; the relay coil and second contact are connected in series with a diode between the line leads.
The supply means in the remote test apparatus comprises a direct voltage source which is selectively connected to the other terminals of the first line section by two transfer contacts. One of the line leads and one of the terminals of the voltage source are always grounded, which calls for a predetermined bias of the test quadrupole. This requirement can generate quadrupole connection errors harmful to operation of the quadrupole during its insertion in the line. This type of test quadrupole is in fact intended for telephone lines peculiar to certain countries, such as North America, where the subscriber line wires, namely the "tip-wire" or T-wire and the "ring-wire" or R-wire, have a predetermined role and cannot be permutated. Such a test quadrupole cannot thus also be used for those telephone subscriber lines in which the order of the two leads is of little importance, as is the case in the French telephone network.
In the first type of test quadrupole disclosed in U.S. Pat. No. 4,054,759, when the relay is activated with a voltage equal to or greater than the predetermined voltage of the remote supply source, the relay responds by operating its first contact such that the relaxation oscillator is looped across the line between the predetermined supply voltage and ground to modulate the subscriber loop current; simultaneously, the subscriber installation that is linked to the other end of the second line section is disconnected. Detection of the modulation frequency in the remote test apparatus indicates the absence of a break in the first line section.
The second type of test quadrupole according to U.S. Pat. No. 4,054,759 also comprises a relay which controls contacts. The relay is interconnected between a ground terminal and a terminal common to first and second diode circuits having other terminals connected to the line leads respectively. Two relaxation oscillators, included in the test quadrupole, operate at different frequencies. The terminals of each oscillator are linked to said ground terminal and to one of the line leads. Such a quadrupole allows each of the line leads to be tested separately by applying a predetermined voltage at the remote end of the first line section to the respective lead in question. When no break of the considered lead exists the respective relaxation oscillator operates in a well known manner to impart an AC modulation component on the current flowing in said lead. The respective modulation frequency is detected in the remote test position. This second type of test quadrupole therefore requires a ground. The modulation frequency return via the ground has the drawback of producing crosstalk in neighbouring subscriber lines that are contained in a same telephone cable. This drawback is hardly compatible with regular preventive tests in subscriber lines.
However, one of the major drawbacks of test quadrupoles according to U.S. Pat. No. 4,054,759 is that the quadrupoles include a ground terminal and cannot indicate insulation faults in the line, such as short circuits between the two line leads or between one of the leads and ground. Furthermore, the use of relays and contacts prevents the possibility of producing quadrupoles in integrated circuit form for the purposes of reducing manufacturing costs and sizes.
Other test quadrupoles for telephone subscriber lines are disclosed in U.S. Pat. No. 3,843,848. Each of these quadrupoles also comprises a relay which, when activated, controls first and second series contacts on the line leads respectively in order to disconnect the subscriber installation and the second line section; here again, faults in the second line section at the subscriber installation end are not taken into consideration.
The activation of test quadrupoles as per U.S. Pat. No. 3,843,848 requires an alternating current trigger signal generator with a predetermined frequency at the remote test end. The quadrupole which comprises solely current detecting means or solely voltage detecting means must include a circuit which has a predetermined inductance and a predetermined capacitance and which is tuned to the predetermined frequency. This requirement generates various distorsions around the predetermined frequency, especially in regard of the impedance, the attenuation and the group propagation time in the line. The distortions are detrimental to the data signals should be subscriber line be digital. In view of the fact that current subscriber lines are for the most part intended in the short term for transmitting digital signals through a modem, the use of this type of quadrupole is very limited.
Moreover, the quadrupoles in U.S. Pat. No. 3,843,848 includes an alternating voltage or current detecting means containing a band pass amplifier and a Schmitt trigger for energizing the relay. A predetermined test tone frequency generator terminates the first line section when the relay is activated in response to the predetermined current or voltage. The generator and Schmitt trigger require a local supply that can be furnished by a battery, needing periodic renewal, or a pair of auxiliary supplying wires that can be accidentally cut.